This invention relates generally to magnetrons of a type using rotating cylindrical sputtering targets, and, more specifically, to structures and techniques for minimizing arcing in such magnetrons.
Cylindrical magnetrons are becoming widely used for depositing films on substrates. An example is the deposition of a stack of dielectric and metal layers on a surface of a glass substrate for the purpose of filtering out a portion of solar energy from passing through the glass. Such a substrate is positioned within a vacuum chamber containing at least one, and usually two, rotating cylindrical targets containing sputtering material on an outer surface thereof. Both inert and reactive gases are generally introduced into the chamber. A voltage applied to the sputtering target, with respect to either the vacuum chamber enclosure or a separate anode, creates a plasma that is localized along a sputtering zone of the target by stationary magnets positioned within the target. Material is sputtered off the target surface and onto the substrate by bombarding the target with electrons and ions of the plasma as it passes through the stationary sputtering zone.
The magnets are usually of a permanent magnet type, arranged along a line within the rotating cylindrical target and held against rotation with the target. The sputtering zone is created by the magnets along substantially the entire length of the cylindrical sputtering target and extending only a small circumferential (radial) difference around it. Traditionally, the magnets are arranged so that the sputtering zone exists at the bottom of the cylindrical target, facing a substrate being coated directly beneath.
Although deposition of the film is desired to take place only on the substrate, it is also deposited on other surfaces within the reactive chamber. This can create a problem in many situations, especially when certain dielectrics are being deposited as the film. For example, if the target surface is silicon or aluminum and the reactive gas is oxygen, silicon dioxide is deposited on the target surface, surfaces of target supporting structures, and the like, as well as on the substrate that is intended to be coated. After a certain build-up of dielectric material on internal vacuum chamber surfaces has occurred over time, arcing to those surfaces can begin. Arcing is undesirable since it generates particles that contaminate the film being deposited on the substrate, and overloads the power supply that creates the plasma through an electrical connection with the sputtering target surface and the vacuum chamber walls or some other anode.
An advantage of a rotating cylindrical sputtering target is that such a film deposited on the target is subjected to being sputtered away as the target surface passes through the sputtering zone, thus counteracting the undesirable film build-up. This is to be contrasted with a planar magnetron sputtering surface that has a fixed sputtering zone, creating a well defined "racetrack" in the sputtering surface, while causing a build-up of an arc causing film on surrounding portions of the planar target. As a result, shields that are used in planar magnetrons to cover such unused sputtering surface portions are believed to be unnecessary in rotary cylindrical magnetrons because of their self-cleaning characteristic. However, undesirable arcing still occurs in rotary magnetrons, under certain circumstances.
Therefore, it is a principal object of the present invention to provide a mechanism and technique for minimizing such arcing.